The potential value of biochar in the mitigation of gaseous emission of nitrogen.

Nitrogen (N) losses through gaseous emission of ammonia (NH3) and nitrous oxide (N2O) can contribute to both economic loss and environmental degradation. This study examined the effect of biochar and a chemical nitrification inhibitor, dicyandiamide (DCD), on N transformation and N losses via gaseous emission of NH3 and N2O from agricultural soils treated with a range of organic and inorganic N sources. The addition of DCD reduced N2O emission from both organic and inorganic N sources treated soils by 75%, but increased ammonium (NH4+) concentration and subsequently induced high NH3 emission from the soils. In contrast, the addition of biochar reduced both N2O and NH3 emissions from organic and inorganic N sources treated soils by 23% and 43%, respectively. The effectiveness of biochar and DCD in reducing NH3 volatilization and N2O emission depends on the nature of the N sources and their initial mineral N concentration. The study demonstrated that biochar can be used to mitigate N losses resulting from NH3 volatilization and N2O emission.

Biochar-induced concomitant decrease in ammonia volatilization and increase in nitrogen use efficiency by wheat.

Ammonia (NH3) volatilization is a major nitrogen (N) loss from the soil, especially under tropical conditions, NH3 volatilization results in low N use efficiency by crops. Incubation experiments were conducted using five soils (pH 5.5-9.0), three N sources such as, urea, di-ammonium phosphate (DAP), and poultry manure (PM) and two biochars such as, poultry litter biochar (PL-BC) and macadamia nut shell biochar (MS-BC). Ammonia volatilization was higher at soil with higher pH (pH exceeding 8) due to the increased hydroxyl ions. Among the N sources, urea recorded the highest NH3 volatilization (151.6 mg kg(-1)soil) followed by PM (124.2 mg kg(-1)soil) and DAP (99 mg kg(-1)soil). Ammonia volatilization was reduced by approximately 70% with PL-BC and MS-BC. The decreased NH3 volatilization with biochars is attributed to multiple mechanisms such as NH3 adsorption/immobilization, and nitrification. Moreover, biochar increased wheat dry weight and N uptake as high as by 24.24% and 76.11%, respectively. This study unravels the immense potential of biochar in decreasing N volatilization from soils and simultaneously improving use efficiency by wheat.

Effects of temperature and amendments on nitrogen mineralization in selected Australian soils.

The effects of temperature (18, 24, and 37 °C) and form of nitrogen (N) input from various sources (organic-green waste compost, biosolids, and chicken manure; inorganic-urea) on N transformation in three different Australian soils with varying pH (4.30, 7.09, and 9.15) were examined. Ammonification rate (ammonium concentration) increased with increase in temperature in all soil types. The effect of temperature on nitrification rate (nitrate concentration) followed 24 > 37 > 18 °C. Nitrification rate was higher in neutral and alkaline soils than acidic soil. Mineral N (bioavailable N) concentration was high in urea treatments than in organic N source treatments in all soil types. Acidic soil lacked nitrification activity resulting in low nitrate (NO3) buildup in urea treatment, whereas a significant NO3 buildup was noticed in green waste compost treatment. In neutral and alkaline soils, the nitrification activity was low at 37 °C in urea treatment but with a significant NO3 buildup in organic amendment added soils. Addition of organic N sources supplied ammonia oxidizing bacteria thereby triggering nitrification in the soils (even at 37 °C). This study posits the following implications: (1) inorganic fertilizer accumulate high NO3 content in soils in a short period of incubation, thereby becoming a potential source of NO3 leaching; (2) organic N sources can serve as possible source of nitrifying bacteria, thereby increasing bioavailable N (NO3) in soils regardless of the soil properties and temperature.

Bioavailability and ecotoxicity of arsenic species in solution culture and soil system: implications to remediation.

In this work, bioavailability and ecotoxicity of arsenite (As(III)) and arsenate (As(V)) species were compared between solution culture and soil system. Firstly, the adsorption of As(III) and As(V) was compared using a number of non-allophanic and allophanic soils. Secondly, the bioavailability and ecotoxicity were examined using germination, phytoavailability, earthworm, and soil microbial activity tests. Both As-spiked soils and As-contaminated sheep dip soils were used to test bioavailability and ecotoxicity. The sheep dip soil which contained predominantly As(V) species was subject to flooding to reduce As(V) to As(III) and then used along with the control treatment soil to compare the bioavailability between As species. Adsorption of As(V) was much higher than that of As(III), and the difference in adsorption between these two species was more pronounced in the allophanic than non-allophanic soils. In the solution culture, there was no significant difference in bioavailability and ecotoxicity, as measured by germination and phytoavailability tests, between these two As species. Whereas in the As-spiked soils, the bioavailability and ecotoxicity were higher for As(III) than As(V), and the difference was more pronounced in the allophanic than non-allophanic soils. Bioavailability of As increased with the flooding of the sheep dip soils which may be attributed to the reduction of As(V) to As(III) species. The results in this study have demonstrated that while in solution, the bioavailability and ecotoxicity do not vary between As(III) and As(V), in soils, the latter species is less bioavailable than the former species because As(V) is more strongly retained than As(III). Since the bioavailability and ecotoxicity of As depend on the nature of As species present in the environment, risk-based remediation approach should aim at controlling the dynamics of As transformation.

Remediation of heavy metal(loid)s contaminated soils--to mobilize or to immobilize?

Unlike organic contaminants, metal(loid)s do not undergo microbial or chemical degradation and persist for a long time after their introduction. Bioavailability of metal(loid)s plays a vital role in the remediation of contaminated soils. In this review, the remediation of heavy metal(loid) contaminated soils through manipulating their bioavailability using a range of soil amendments will be presented. Mobilizing amendments such as chelating and desorbing agents increase the bioavailability and mobility of metal(loid)s. Immobilizing amendments such of precipitating agents and sorbent materials decrease the bioavailabilty and mobility of metal(loid)s. Mobilizing agents can be used to enhance the removal of heavy metal(loid)s though plant uptake and soil washing. Immobilizing agents can be used to reduce the transfer to metal(loid)s to food chain via plant uptake and leaching to groundwater. One of the major limitations of mobilizing technique is susceptibility to leaching of the mobilized heavy metal(loid)s in the absence of active plant uptake. Similarly, in the case of the immobilization technique the long-term stability of the immobilized heavy metal(loid)s needs to be monitored.

Role of organic amendment application on greenhouse gas emission from soil.

Globally, substantial quantities of organic amendments (OAs) such as plant residues (3.8×10(9) Mg/yr), biosolids (10×10(7) Mg/yr), and animal manures (7×10(9) Mg/yr) are produced. Recycling these OAs in agriculture possesses several advantages such as improving plant growth, yield, soil carbon content, and microbial biomass and activity. Nevertheless, OA applications hold some disadvantages such as nutrient eutrophication and greenhouse gas (GHG) emission. Agriculture sector plays a vital role in GHG emission (carbon dioxide- CO2, methane- CH4, and nitrous oxide- N2O). Though CH4 and N2O are emitted in less quantity than CO2, they are 21 and 310 times more powerful in global warming potential, respectively. Although there have been reviews on the role of mineral fertilizer application on GHG emission, there has been no comprehensive review on the effect of OA application on GHG emission in agricultural soils. The review starts with the quantification of various OAs used in agriculture that include manures, biosolids, and crop residues along with their role in improving soil health. Then, it discusses four major OA induced-GHG emission processes (i.e., priming effect, methanogenesis, nitrification, and denitrification) by highlighting the impact of OA application on GHG emission from soil. For example, globally 10×10(7) Mg biosolids are produced annually which can result in the potential emission of 530 Gg of CH4 and 60 Gg of N2O. The article then aims to highlight the soil, climatic, and OA factors affecting OA induced-GHG emission and the management practices to mitigate the emission. This review emphasizes the future research needs in relation to nitrogen and carbon dynamics in soil to broaden the use of OAs in agriculture to maintain soil health with minimum impact on GHG emission from agriculture.